Laser diodes having high output powers are principally produced as so-called broad stripe lasers. In this case, the amplification of the optical field takes place in an active layer, which contains a quantum well structure, for example. The active layer of the semiconductor laser is typically embedded in waveguide layers in which the laser modes can propagate. The performance of the semiconductor laser is limited by the power density of the laser modes in the facet region. Excessively high peak field strengths at the side facet lead to the melting of the semiconductor material and to the destruction of the semiconductor laser. This damage to the laser is referred to as COMD (Catastrophic Optical Mirror Damage).
By virtue of a large lateral extent of the active layer, a high optical power can be coupled out from the edge emitter, without the critical power density being exceeded. As the active layer is widened, however, the number of transverse modes which can be amplified in this direction in the waveguide also increases, which results in a deterioration in the beam quality of the laser radiation coupled out.
For most applications of semiconductor lasers, operation in the transverse fundamental mode (single-mode laser) is desirable since the intensity profile of the lateral fundamental mode facilitates beam shaping and, in particular, the coupling of the laser radiation into optical waveguides. Moreover, the maximum power of the semiconductor laser can be increased in this case since the fundamental mode typically has no pronounced intensity peaks.
During the amplification of the desired lateral fundamental mode in the waveguide of the semiconductor laser, the amplification of the semiconductor material is selectively reduced locally, in particular, in a central region of the waveguide. On the other hand, the amplification still remains high in the edge regions of the waveguide. This amplification can suffice to cause higher modes to commence oscillation. Experimental investigations on broad stripe lasers show that the intensity distributions of the emitted radiation tend toward power boosting in the outer region of the active zone. This observation can be explained by the superposition of higher modes that experience a high degree of amplification in the outer region of the waveguide.
One approach for shaping the laser modes in semiconductor broad stripe lasers involves introducing so-called phase structures. This is described in the document WO 01/97349 A1.